Clean up `topics` vs `practices` in `config.json`

Per the [current docs](https://exercism.org/docs/building/tracks/config-json), the track-level `config.json`'s per-exercise `topics` key isn't used for anything. This has the advantage that the items there aren't constrained by cross-checks, but the disadvantage that we're not fully exposing to the website the information we have about each exercise.

The general procedure we want to follow goes like this:

- Collect the list of all current concept exercises. Currently:
```bash
$ jq -r '.concepts[] | .slug' config.json | sort -u
booleans
box
char
collect
destructuring
entry-api
enums
floating-point-numbers
fold
functions
hashmap
integers
loops
match-basics
mutability
option
references
strings
string-slices
string-vs-str
structs
tuples
vec-macro
vec-stack
````
- For each exercise among the practice exercises:
- For each item in `topics`
- If the topic is a current concept, move it from `topics` to `practices`

This process needs to be repeated periodically as we add concepts, so PRs addressing this issue should not close it.